This invention relates to a process for purifying argon gas in a waste gas and an apparatus used therefor.
In order to purify argon from a waste gas, since combustible constituents contained in the waste gas are combusted with oxygen, the waste gas is contaminated with a large amount of residual oxygen. In this case, separation of oxygen and argon has been disadvantageous in that since their boiling points are as similar as -182.97.degree. C. and -186.0.degree. C. (atmospheric pressure), respectively, a large rectifying tower is necessary for separating them by reactification.
A silicon furnace is a furnace used for producing silicon by withdrawing single crystal silicon from melted crude silicon with the aid of a seed, and for the production, argon is used as atmosphere gas.
Argon is contained in a high concentration of 96 to 98% in a waste gas composition exhausted from the silicon furnace, and is reused after being purified and recovered from said gas composition because argon is expensive.
As such a process for purifying argon, there is disclosed, for example, a process which, as shown in Japanese Patent Appln Kokai (Laid-Open) No. 72,394/77, comprises passing impurities in argon such as carbon monoxide, hydrogen, tritium, carbon dioxide, nitrogen, oxygen, methane and acetylene through a first reaction tube packed with a metal catalyst such as palladium, platinum or the like to react oxygen with impurities among them which are active toward oxygen, adding, to the gas which has come out of the first reaction tube, hydrogen in an amount excessive over the amount of oxygen in said gas, passing the resulting gas through a second reaction tube to effect reaction between the oxygen and hydrogen in the gas, passing the gas through an adsorption tower to remove the water and carbon dioxide in the gas by adsorption, and then passing the residue through a low-temperature liquefaction separation tower to purify and separate the same into hydrogen, nitrogen and argon.
Such a purification process is disadvantageous in that the constitution of apparatus is complicated because since said process requires subzero treatment at about -200.degree. C., there are conducted, as pretreatments, removal of hydrocarbons such as methane, acetylene and the like, removal of water and carbon dioxide for preventing their condensation and solidification in the pipes of an apparatus for subzero treatment, and removal of oxygen similar to argon in boiling point for improving the efficiency of argon recovery.
Further, there is disclosed a process which, as shown in Japanese Patent Publication No. 28,750/77, comprises passing impurities in argon such as carbon monoxide, carbon dioxide and oxygen through a combustor to combust carbon monoxide with oxygen into carbon dioxide, then carrying out subzero treatment at about -100.degree. C., thereby separating the carbon dioxide to purify and separate argon gas.
Since this process is intended for an argon-oxygen decarbonization hearth, the presence of oxygen in the recovered gas is allowed up to about 30%, so that removal of oxygen is not particularly necessary. However, this process is disadvantageous in that nitrogen and water should be removed and that removal of carbon dioxide requires subzero treatment at about -100.degree. C., so that the apparatus becomes complicated.
On the other hand, removal of oxygen by addition of hydrogen is disclosed, for example, in U.S. Pat. Nos. 3,535,074 and 4,299,800 and East German Pat. No. 154,958; removal of hydrocarbons is disclosed, for example, in U.S. Pat. Nos. 3,717,572; 4,025,321; and 4,257,885, removal of oxygen by addition of hydrocarbons is disclosed, e.g., in U.S. Pat. Nos. 3,878,289; 4,156,461; and 4,505,880, removal of trace oxygen with heating is disclosed, for example, in U.S. Pat. No. 3,996,028, and removal of hydrogen by addition of oxygen is disclosed, for example, in U.S. Pat. No. 4,459,270. But these references show only special steps for attaining their objects and are quite silent on combinations of these steps.